Michael A. Wilson Lincoln University College Conterbury, New Zealand
I
I
Classification of the Electrophilic Addition Reactions of Olefins and Acetylenes
Modern organic chemistry is often taught on a mechanistic basis in which reaction types are classified along the lines initially introduced by Ingold (I). Thus the symbolE2H, etc. are well known and freisms S N ~S, N ~ElcB, , quently used among graduates and undergraduates alike. However, this could not be said to be true of the descriptions of electrophilic addition reactions where lack of knowledge has hindered the general introduction of similar terminology and it is the writer's opinion that lecturers who are not researching in this area usually approach this subject, and teach it, a little hazily. Recently, with the renewed interest stimulated by several reviews in the field of electrophilic addition a clearer picture of the mechanisms of these reactions has been revealed. This article hopes to bring these facts to the attention of people not directly involved in research in this area, and thereby produce a more solid basis on which this subject can be introduced. In describing various types of chemical reactions i t is useful to group together those processes which have properties in common. The term addition has been used 12) to describe reactions in which the products are molecules in which both atoms of the unsaturated bond have become saturated by the formation of covalent bonds. Using this definition, the reaction of a hypothetical molecule XY with an alkene (I) to give an adduct (11) is an addition reaction
free energy profile which describes the course of the reaction. This definition includes those processes which form ring compounds either inter or intramolecularly. The addition process may be synchronous or multistep. In the latter case alternative products may arise from intermediates along the reaction path. Both adducts of XY and any other products from the same intermediates have part of their reaction pathways in common. Consequently, a classification based on initiation of reaction has merit since it encompasses these variations. Thus Ingold (1) distinguishes between bomolytic processes involving the donation of one electron from both reagent and substrate to form a covalent bond, and heterolytic processes in which pairs of electrons from substrate or reagent are transferred. These latter processes may be electrophilic or nucleophilic, i.e. electrons may be transferred to or from the reagent, respectively, in the initiation step. Ingold (1) also distinguished cyclic additions which proceed through a concerted transition state in which both forming bonds are equally developed. These cannot be classified as homolytic or heterolytic because there is no way of telling structures (ID), (N), and (V) from eachother.
X Y
(1) (II) Whether (11) reacts further to give some other product may be of interest, but (II) must be a minimum on the
Such a process will have no development of charge in the transition state relative to the initial state, and will thus be insensitive to changes in reaction parameters such
Some Examples of Electrophilic Addition Processes Substrate
Eleetrophile
DOAc
HOAc HC1 HCI CI? ClOAc
(eoneentratio" *1 X 10.8 mole
Mechanism
Styrene Methyl cinnsmate Methyl cinnamate Ally1 chloride Styrene
HOAc HOAc HOAc HOAc HOAc HOAe HOAc
cis-2-Butene
HOAc
Stepwise Bimolecvlar Eleetmphilic
trans-2-Butene
Stepwise Bimokular Electrophilie Stepwise Bimolecular Electmphilie Termoleeular Electrophilic Addition Bimolecular Eleetrophilic Addition Termolecular Electrophdie ~dditi~., B ~ ~ ~ I Electrophilic Addition Termolecular Electrophilic Addition
t-Butylethylene
1.1)
Solvent
SO* (I),CHClr EtOH, CHCh
Molecular Molecular Molecular (+ very small amount of Ad&) Molecular (+ very small amount of AdgW Molecular Ad62 Stepwise Bimolecular Electrophilie Stepwise Bimolecular Electrophilie Stepwise Bimolecular Eleetrophilie Stepwise Bimol-lar Electrophilie Stepwise Bimolecular Electrophilie Stepwise Bimolecular Electrophilic
Norbomene Norbomadiene Norbomadiene Norbarnadiene Norbornadiene
NOCl NOCl HOAc
Nonpolar Nonpolar
(NO?)%CIHSCI HCI
Styrene Cyeloheaene
HOAc HOAe HOAc
HCI
1.2-Dimethylcyclohe.ene
HOAc
Brt (concentration 5 X 10-"molel-~
Methylphenylacetylene
HOAe
+ Br-O.lmolel-') Bn (concentration -1 x 10-1 mole 1-1)
Styrene
HOAe
ICL
Ethyl
EtCOOH
butyned-oate
+
Ad~2 Ad~2
Ade3 +Ad&
+
+
~
Adz3
~ + A~~ E Z I
(32) (33)
(34) (35)
~
Add
(27)
Ter~nolecularElectrophilic Addition but with attack of the semnd molecule of electrophile a t bromine
Ad&E
(24)
Tetramolecular Electrophdic Addition
Ade4
(36)
~
Volume 52, Number 8, August 1975 / 495
as solvent which alter polarity. It can he represented as
.Cycloadditions leading to cyclic compounds have been
1x71) \ -1.
reviewed by Huisgen (3) and recently by Schmidt (41. It has been pointed out by de la Mare (2) that the term cycloaddition should not exclude those processes in which a ring structure is formed only in the transition state, i.e. structures (VII), (VIII), (IX), jointly represented as (X), because they are obviously related to (VI). Both mechanisms have, for example, similar stereochemistries. Indeed, many electrophilic or nucleophilic processes could conceivably be very similar.
(m
(W) (vm) (X) In the case of electrophilic processes the analogous reactions have been termed molecular additions 15). If the nucleophile arises from the incoming electrophile and in the slow step a cyclic transition state is formed, then the simplest possible mechanism for an electrophilic pathway is similar to structure (X). Both the C-X and C-Y bonds are forming in the transition state. The X-Y hond may he breaking a t the same time. Fahey (5) draws this transition state as in (XI) and Traylor (6) as in (XII) and (XIII) for additions to some bicyclic systems
There must be some charge-separation in the transition state because of the effect of electron withdrawing substituents on the rate (6). i.e. for the process to be electrophilic; and the C-X hond must be more fully formed than the C-Y bond. Depending on the electrophile, there may or may not he interaction of X with the electron-deficient center. For these processes the distinction between nucleophilic (e.g. (XIV)) and electrophilic may lie a t opposite ends of the spectrum from the concerted process (X) /
Molecular Addition (in which molecular complexes may or may not be involved).
Stepwise Addition (in which ionic intermediates are farmed. Molecular complexes may or may not be farmed as well)
discussed above. The stereochemical implication of this mechanism is syn-addition. However, there is no reason, in principle, to suggest that this mechanism could not be extended to transition states with more than four centers, although for large rings the entropy of activation may seriously disfavor these processes. Six-centered transition states may then lead to syn-(XV) (7) or even anti-addition (XVI) (8)for more complex electrophiles (hypothetically, X-A-B==Y).
1 (XV) (XVI) Molecular addition is believed to be common in bicvclic systems because, as Traylor has pointed out (6),the alternative mechanisms leadine to anti-addition are seriouslv disfavored because the required anti-coplanar arrangement of atoms can be achieved only a t the expense of bond-angle strain in the ring and with the introduction of steric repulsion. Arguments for the importance of molecular addition are based on the lack of or, comparatively with solvolysis, decrease in rearrangement in these systems (7), which are known to undergo extensive rearrangement on formation of a discrete carbonium ion, and also on the importance of bulky groups sterically hindering approach to the double bond (9).Oxymercuration of hicyclic systems was believed to proceed via a molecular process but this generally is now not believed to be correct (9). For the majority of electrophilic addition reactions investigated to date, there is considerable evidence that energy minima exist along the reaction profile which describes the course of these reactions. When two molecules approach each other, induced dipole or dipole-dipole interactions result in attractions between these molecules and are termed van der Waal interactions. These forces have been found largely responsible for complex formation between olefins and a variety of compounds (10, l l ) , although charge-transfer bonding may make a small contribution to the stability of these complexes (11). Such complexes have been termed molecular complexes, in particunucleophilic concerted
, electrophilic
initial qtep nucleophilic
\initial
Rate limiting attack step -of the electmphile --Ad& electrophilic an the olefin
I
Rate limiting attack of the nucleo~hileon the intermediate
Termolecular Additions (attack of nueleaphile and electrophile at the same time from different molecules) Differentiation of addition processes 496 / Journal of ChemicalEducation
electrophilie nucleophilic
lar to distinguish them from other z-complexes (12). Apart from these entities i t has often been demonstrated (13) that other intermediates other than molecular complexes exist along the reaction coordinate to products. Thus rearrangements may be ohsewed or when other nucleophiles than those brought by the incipient electropbile are included in the reaction medium, capture may take place to form alternative products. The origin and role of the nucleophile in the pathway to products have been used to differentiate the addition process further (1, 5, 13) (see figure, p. 496). The A d ~ 2and Ad83 processes areillustrated in eqns. (1) 431, respectively. In general A d ~ 3processes show thirdorder kinetics and A d ~ 2processes a second-order kinetic form. However, for an A d ~ 3pr'ocess the possibility remains that if the nucleophile Y- is derived reversibly from the original electrophile XY, then the kinetics may be second-order. Similarly, if the nucleophile arises from solvent, the kinetics may he second-order. Strictly speaking an electrophilic molecular addition is also an AdE2 process since it is bimolecular. A suggested terminology might he A ~ E ~ M It . can be seen from eqns. (1)-(3) that stereochemistry in itself is no guide to mechanism. Both molecular addition, stepwise addition, and termolecular addition may lead to syn-addition adducts. Similarly stepwise and termolecular processes may give rise to anti-addition products; the electrophilic group 'X' may bridge t o form a bridged ion e.g. the hromonium ion.
The possibility also exists that a spectrum of transition states from nucleophilic through concerted to electrophilic may exist for both termolecular and molecular processes. Thus some additions may he better classified for examole as Ad3 or AdN3. Dubois (141 has introduced the terms Ad&2 and Ad&1 to describe the processes which involve complex formation along the reaction pathway. Thus an Ad&l process is one in which a prior formed complex decompos-
es unimolecularly to a further intermediate or product along the reaction pathway. Solvent may assist in rupture of the X-Y bond. An AdEC2 process involves attack of a further molecule of reagent on the olefin-electrophile complex. In hydration and related fields yet another terminology has been used. In an A-1 mechanism the olefin is protonated in a fast step but only the protonated substrate is involved in the subsequent rate determining stage. In an A-2 mechanism the protonated substrate undergoes rate determining attack of nucleophile. The symbol A-SE2 has also been used to imply rate determining attack of the proton on the olefin. Most of the evidence to suggest prior coordination of protons to form n-complexes is now believed to be unsubstantiated, and these pathways can also he grouped under the headings, AdE3and A d ~ 2 . It is very difficult to obtain unambiguous evidence that complexes are actually involved along the reaction path and that sequences such as eqn. (3) are not involved.
Thus the evidence for c o m ~ l e xformation in electro~hilic addition reactions is largel; circumstantial, althoug6 correlations between rate and spectroscopic constants measuring the stability of such constants have been established (15, 16) and plausible explanationsofsolvent and substituent effects on rates have been formulated in terms of these pathways. Addition of hydrogen chloride to isobutylene in aprotic solvents involves kinetic terms of high order and is faster if cooled and allowed to warm to room temperature than if the reaction is started a t room temperature. This was suggested as evidence for some sort of complex formation (18). Dewar (19) pointed out that this explanation would he unsatisfactorv if a hieh enerev -" intermediate was involved since a single step multimolecular addition is indistinguishable from one of manv bimolecular steps if these steps occur before the rate determining stage. This applies only when the concentration of intermediate is negligible and when an equilibrium (20) or steady state (20) approach is also used. The intermediates are highly reactive and their concentration is small and changes very little with time. For halogenation, similar evidence comes from Buckles and Womers work (1 7). The equilibrium constant for formation of a blue complex increases on lowering the temperature, and bromine is more easily removed from this complex by copper than from the product of the reaction. Recently Gebelein and Frederick (21) have offered kinetic evidence of a different type. The kinetics of bromine addition in nonpolar solvents show extensive experimental scatter (22). However, Gebelein and Frederick have obtained reproducible thirdorder kinetics for the hromination of (2)-3-hexene and (Eb3-hexene. These workers have found that the rate constant varies widely when the concentration of reactants is changed. Using a steady state or equilibrium approach they.attribute this variation to the effect of the finite concentration of molecular complex on the concentration of the other reacting species. Although the concentration of this complex was unknown and thus the real value of the rate constant was incomputable, the integrated rate equaV O I U 52, ~ ~Number 8, August 1975 / 497
tion shows that the error introduced through ignoring this concentration should vary in a systematic manner. This prediction was in agreement with the experimental results although there was much scatter. The magnitude of the ratio of equilibrium constants obtained by extrapolation from a competitive method was acceptable when comoared with the known s~ectronhotometrical~v determined equilibrium constants for similar complexes with iodine. In addition to the ~ r o b l e mof ex~erimentallyobserving the involvement of complexes along reaction paths, there has been some confusion over the term 'complex' in these reactions as well (10). Thus the complex may be described as a molecular complex, an aggregate held together predominantly by weak van der Waal forces or a u-complex which contains a considerable amount of covalent bonding. In this latter example even carbonium ions have been formulated as x-complexes in some cases (10); so it is difficult to describe an aggregate along the reaction coordinate without really being concemed- with the bonding in that intermediate. In this context i t is relevant to note that charge-transfer spectra have been observed for aggregates of encounter lifetime only (10) and any small energy minimum, including those involving only solvent reorientation might be described as complexes which exist along the reaction coordinate. Consequently, the terms Ad&2 and AdECl have been little used as 'handles' in elucidating reaction mechanisms. What the work of Dubois (14-16) and others (23-26) has shown however is that the routes to products along the reaction pathway leading to addition may involve multiple intermediates all of which may collapse to products a t different rates. In termolecular processes i t has become evident that addition may not be only in the strict AdE3 sense as seems likely for the bromide ion catalyzed addition of bromine to methylphenylacetylene (XVII) (27) Ph Br - , ~ r ' B;
'
~
e
(XW) but that attack may occur of the second molecule of reagent a t the adjoined electrophile (XVIII).
x-px-Y
7Y
I-' x-
r* x-Y xY -
H 7Y
(XVIII) There may then be room for a term to describe this mode of attack irrespective of what the species involved represent. Possibly, by analogy with elimination where the modes EZH, EZC, describe the positions of attack, then the term AdE3E might be acceptable, since the 'E' refers to attack a t the electrophile by the second molecule of reagent.
498 / Journal of ChemicalEducation
To conclude, there are several mechanistic pathways under which electrophilic addition may be taught. They are listed in the table together with some good teaching examples. However, i t is unlikely that even this encompasses all the mechanistic variations. Greater than thirdorder kinetics have been known for some time in many electrophilic processes (13). Recent workers have suggested that these may be the result of complexing of two or more electrophiles to the olefin, before the rate determining step, and an A d ~ 4mechanism has thus been proposed (36). What has become clear, however, is that a progression of pathways exists, and many additions involve a number of competing reactions for both olefins and intermediates.
Lilerature Cited Ingold. C. K.. "Structure and Mechanism in Orgenic Chemistry," 2nd Edition, G. Bell and sons. London. 1969 de la Mare. P. B. D.. Sci. Plaar. fLondan1.56.243 119681. Huisgcn, R.,Angew. Chem flnf Edition], 2. 565 l19611. Schmidt. R. R.. Angem. Chcm l l n f Edcrion,, 12,212 (1973) Fahey. R. C.. TooicsS~emoehem..3.237 119681. nayiar,~.~.,Acountschem R&, . 2,ibz 119691. la) Cristal, S. J., Mnrill. T . C.. and Sanchez, R. A,, J. Urg. Chem., 31, 2719, 2726, 2733 (1966): l b l J. Am
